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io_uring: fix poll_list race for SETUP_IOPOLL|SETUP_SQPOLL

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After making ext4 support iopoll method:
  let ext4_file_operations's iopoll method be iomap_dio_iopoll(),
we found fio can easily hang in fio_ioring_getevents() with below fio
job:
    rm -f testfile; sync;
    sudo fio -name=fiotest -filename=testfile -iodepth=128 -thread
-rw=write -ioengine=io_uring  -hipri=1 -sqthread_poll=1 -direct=1
-bs=4k -size=10G -numjobs=8 -runtime=2000 -group_reporting
with IORING_SETUP_SQPOLL and IORING_SETUP_IOPOLL enabled.

There are two issues that results in this hang, one reason is that
when IORING_SETUP_SQPOLL and IORING_SETUP_IOPOLL are enabled, fio
does not use io_uring_enter to get completed events, it relies on
kernel io_sq_thread to poll for completed events.

Another reason is that there is a race: when io_submit_sqes() in
io_sq_thread() submits a batch of sqes, variable 'inflight' will
record the number of submitted reqs, then io_sq_thread will poll for
reqs which have been added to poll_list. But note, if some previous
reqs have been punted to io worker, these reqs will won't be in
poll_list timely. io_sq_thread() will only poll for a part of previous
submitted reqs, and then find poll_list is empty, reset variable
'inflight' to be zero. If app just waits these deferred reqs and does
not wake up io_sq_thread again, then hang happens.

For app that entirely relies on io_sq_thread to poll completed requests,
let io_iopoll_req_issued() wake up io_sq_thread properly when adding new
element to poll_list, and when io_sq_thread prepares to sleep, check
whether poll_list is empty again, if not empty, continue to poll.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Xiaoguang Wang 2020-02-25 22:12:08 +08:00 committed by Jens Axboe
parent 41726c9a50
commit bdcd3eab2a
1 changed files with 27 additions and 32 deletions
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59
fs/io_uring.c
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@ -1821,6 +1821,10 @@ static void io_iopoll_req_issued(struct io_kiocb *req)
list_add(&req->list, &ctx->poll_list);
else
list_add_tail(&req->list, &ctx->poll_list);
if ((ctx->flags & IORING_SETUP_SQPOLL) &&
wq_has_sleeper(&ctx->sqo_wait))
wake_up(&ctx->sqo_wait);
}
static void io_file_put(struct io_submit_state *state)
@ -5086,9 +5090,8 @@ static int io_sq_thread(void *data)
const struct cred *old_cred;
mm_segment_t old_fs;
DEFINE_WAIT(wait);
unsigned inflight;
unsigned long timeout;
int ret;
int ret = 0;
complete(&ctx->completions[1]);
@ -5096,39 +5099,19 @@ static int io_sq_thread(void *data)
set_fs(USER_DS);
old_cred = override_creds(ctx->creds);
ret = timeout = inflight = 0;
timeout = jiffies + ctx->sq_thread_idle;
while (!kthread_should_park()) {
unsigned int to_submit;
if (inflight) {
if (!list_empty(&ctx->poll_list)) {
unsigned nr_events = 0;
if (ctx->flags & IORING_SETUP_IOPOLL) {
/*
* inflight is the count of the maximum possible
* entries we submitted, but it can be smaller
* if we dropped some of them. If we don't have
* poll entries available, then we know that we
* have nothing left to poll for. Reset the
* inflight count to zero in that case.
*/
mutex_lock(&ctx->uring_lock);
if (!list_empty(&ctx->poll_list))
io_iopoll_getevents(ctx, &nr_events, 0);
else
inflight = 0;
mutex_unlock(&ctx->uring_lock);
} else {
/*
* Normal IO, just pretend everything completed.
* We don't have to poll completions for that.
*/
nr_events = inflight;
}
inflight -= nr_events;
if (!inflight)
mutex_lock(&ctx->uring_lock);
if (!list_empty(&ctx->poll_list))
io_iopoll_getevents(ctx, &nr_events, 0);
else
timeout = jiffies + ctx->sq_thread_idle;
mutex_unlock(&ctx->uring_lock);
}
to_submit = io_sqring_entries(ctx);
@ -5157,7 +5140,7 @@ static int io_sq_thread(void *data)
* more IO, we should wait for the application to
* reap events and wake us up.
*/
if (inflight ||
if (!list_empty(&ctx->poll_list) ||
(!time_after(jiffies, timeout) && ret != -EBUSY &&
!percpu_ref_is_dying(&ctx->refs))) {
cond_resched();
@ -5167,6 +5150,19 @@ static int io_sq_thread(void *data)
prepare_to_wait(&ctx->sqo_wait, &wait,
TASK_INTERRUPTIBLE);
/*
* While doing polled IO, before going to sleep, we need
* to check if there are new reqs added to poll_list, it
* is because reqs may have been punted to io worker and
* will be added to poll_list later, hence check the
* poll_list again.
*/
if ((ctx->flags & IORING_SETUP_IOPOLL) &&
!list_empty_careful(&ctx->poll_list)) {
finish_wait(&ctx->sqo_wait, &wait);
continue;
}
/* Tell userspace we may need a wakeup call */
ctx->rings->sq_flags |= IORING_SQ_NEED_WAKEUP;
/* make sure to read SQ tail after writing flags */
@ -5194,8 +5190,7 @@ static int io_sq_thread(void *data)
mutex_lock(&ctx->uring_lock);
ret = io_submit_sqes(ctx, to_submit, NULL, -1, &cur_mm, true);
mutex_unlock(&ctx->uring_lock);
if (ret > 0)
inflight += ret;
timeout = jiffies + ctx->sq_thread_idle;
}
set_fs(old_fs);